Resonant effects in dielectric gratings were identified in the early 1990's as having promising applications to free-space optical filtering and sensing. Resonant effects typically occur in sub-wavelength gratings, where the first-order diffracted mode corresponds not to freely propagating light but to a guided wave trapped within a dielectric layer. The trapped wave is re-scattered in the 0th diffracted order and interfaces with the incident light to create a pronounced modulation of transmission and reflection. When a high-index-contrast grating is used, the guided waves are rapidly scattered and do not propagate very far laterally. As a result, the resonant feature can be considerably broadband and of high angular tolerance, which has been used to design novel types of highly reflective mirrors. Recently, sub-wavelength grating mirrors have been used to replace the top dielectric stacks in vertical-cavity surface-emitting lasers, and in novel micro-electromechanical devices. In addition to being more compact and relatively cheaper to fabricate, sub-wavelength grating mirrors also provide polarization control.
However, when designing sub-wavelength diffraction grating devices operating in a reflective mode, especially when a high degree of reflectivity is desired, the phase response of the reflective device may be difficult to control. Thus, although the device may possess the desired reflectivity, the inability to achieve the desired phase response can cause the performance of the device to suffer. In one example, a reflective device may exhibit high reflectivity of light arriving near the axis of the device but may not be able to focus light received over wide angles. In such an instance, only a small amount of light arriving from oblique angles can be directed to the focal point of the device.